User operated drug delivery devices are as such known in the art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicament, such as heparin or insulin. In particular, such devices have application, where a medicament is administered on a regular or irregular basis over a short-term or long-term period.
In order to accommodate with these demands, such devices have to fulfil a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. The dose setting must be easy and unambiguous. Where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose.
Some mechanically implemented drug delivery devices, e.g. pen-type injectors, comprise a dosing or dose setting arrangement featuring a dose setting member to be displaced in axial proximal direction for setting of a dose. For dispensing of a respective dose, the proximally extended dose setting member may then have to be manually depressed in distal direction, e.g. to advance a piston of the cartridge in distal direction. Here, a user may have to exert a comparatively large dispensing force, which may pose a burden to physically impaired persons or patients.
Apart from manually or purely mechanically operated drug delivery devices there also exist electro-mechanical drug delivery devices wherein at least dose setting, dose selection or administration of an appropriate dose of the medicament is controlled by way of electronic circuitry, for example a controller, a microprocessor, and/or the like. Such electronic or electro-mechanical devices provide a high dosing accuracy and may support long-term monitoring of doses dispensed by the device. Hence, a dosing and dispensing scheme can even be stored in the device allowing to recall the dose dispensing history later on.
However, with electronic or electro-mechanical devices, setting and dispensing of a dose might be less intuitive compared to an all-mechanically implemented device. In particular, with a mechanically implemented device, the user has to apply a certain injection force, thereby obtaining a force feedback. By introducing an electro-mechanical device to the patient and replacing an all-mechanical device, the patient or user may be confronted with an unfamiliar or inconvenient operation scheme. Hence, there may emerge a certain danger of misuse and suboptimal medical treatment. Also, the acceptance of such an electro-mechanical device by the user may be rather low.
With an electronic or electromechanical drug delivery device not only the dose setting and dose dispensing process is electrically implemented. Also a dose indicator, typically in form of a display to visually illustrate the size of a dose to the user may be implemented all electronically. However, in the event that the drug delivery device or one of its electronic components becomes subject to failure, there may be a certain danger, that the dose is incorrectly or incompletely indicated to the user of the device. It may then occur that the indicated or illustrated dose size does not match with the size of a dose actually set or dispensed by the drug delivery device.
It is therefore an object of the present invention to provide an improved drive mechanism for a drug delivery device, in particular for a pen-type injector. The drive mechanism should be mechanically operable and should provide direct and intuitive feedback to the user during dose dispensing. The drive mechanism should further come along with a substantially reduced dispensing force required to be exerted by the user for dose injecting or dose dispensing. In that sense the drive mechanism should provide a rather easy and intuitive handling, both for setting as well as for dispensing of a dose. Moreover, the invention also relates to a drug delivery device comprising such improved drive mechanism.